Industry is making changes to reduce the properties of casting and is taking steps to minimize the splashing,reoxidation and bubble trails by calculations of the velocities of metal.
The result of the progress is that the hot box bonded cores were converted into Cold box.It helped in reducing the core costs produced and gave a significant production rate. Cold-box method kept developing for better gas emissions and scrubbing equipment for core removal..
Cold box core making helped in building a computer-based engineering design and is very useful as all the softwares in the industry of solidification etc are up to date.
The tool designing helps a lot in the field for the core geometry,core boxes,blow tube selection, vent selection and location.Processes can be modeled and optimized.A suitable set of process can be created by choosing gassing equipment,core-making, resin, catalyst etc
The cores should be removed easily and should be weak so that it could stop the casting shrinkage when it cools and avoids cracking.In the end,cores can be removed easily by shaking.
There should be no gases involved in the casting and the core material has to be removed to provide the specified cavity so the cores must be connected to the surfaces that are outside the mold cavity.At these connection,there are core prints that are used to hold the cores in their respective position in the process of mold filling.
ANSWER (c) :
The first step is to size the vents. The vent area are often calculated supporting the thought that the air has got to leave the cavity at an equivalent flow because the flow of the metal entezing the cavity. The incoming metal flow rate is Q, which can be calculated a number of ways, but it usually is calculated from the plunger speed th-tes the plunger area. The desirable minimum area of the vents can be found from:
Area of vents = Q/VMAX
Area of vents = The area of all the vents on the whole die
Q = Flow rate of the metal into the cavity
VMAX — Maximum air velocity within the vents, which is usually recommended to be: 8000 ips or 200 m/sec
The maximum air velocity within the vents is taken as 8000 ips (200 m/ sec), which is about 70% of the speed of sound at standard conditions. This is judged to be the.moamum practical speed in the vents — above that speed the back pressure builds up and air flow does not increase because of the shock wave affect. This formula will often end in a vent area of about 20% to 25% of the gate area, assuming a design gate velocity of about 1600 ips to 2000 ips.
The next step is getting the vents in the best location. Most vents should be located at the last points to fill, and finding these locations can be done with experience using the blind locanons at the end of the casting; or with a simulation.
Vents must go to the edge of the die to be effective. Vents usually are about .004 to .006 deep for ztnc and magnesium, and .005 to .008 deep for aluminum. The vents must be polished to allow any metal in the vents to come out with the shot. Draft is needed on the sides (even if they are only .005 inches deep) to keep any metal from sticking. It is always tempting to the operator to go away metal hanging within the vents if it's hard to wash out — in any case , the casting doesn't look any different.
Using a "Z" shaped vent can allow the vent to be deeper (up to about .03 Inches in some cases):
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